Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A PROCESS FOR MAKING RARE EARTH DOPED OPTICAL FIBRE
Technical Field
The present invention relates to a Process for Making Rare Earth Doped Optical
Fibre.
Background Art
Rare-earth (RE) doped optical fibres have shown great potential for a number
of
applications including amplifiers, fibre lasers and sensors. Oxides of rare
earths are doped
into the core of such fibres as the active substance. Lasing and amplification
have been
demonstrated at several wavelengths with the incorporation of various rare-
earths but for
1o telecommunication applications erbium doped fibre (EDF) remains the most
important
since the operating wavelength matches with the third low loss optical window.
Erbium doped fibre amplifier (EDFA) operating around 1.53 ~,m low loss window
is
playing the key role in the present day high capacity communication systems.
It is able to
amplify the optical signal directly independent of modulation format.
Optoelectronic
repeaters so long used in these systems were 3R devices with the limitations
of amplifying
the signal in discrete wavelengths. EDFA has the capability to amplify
simultaneous
optical channels in a single fibre, which has enabled the implementation of
WDM
(wavelength division multiplexing) technology with the potential of increasing
the
bandwidth of long distance transmission systems from Gb/s to Tb/s ranges. It
thus exhibits
2o high gain, large bandwidth, low noise, polarisation insensitive gain,
substantially reduced
cross talk problems and low insertion losses at the operating wavelengths. The
success of
future high capacity optical networking and transmission systems will depend
significantly
on the development of efficient EDFA.
Reference may be made to Townsend J.E., Poole S.B., and Payne D.N.,
Electronics
Letters, Vol. 23 (1987) p-329, ' Solution-doping technique for fabrication of
rare-earth-
doped optical fibre' wherein, the MCVD process is used to fabricate the
preform with a
step index profile and desired core-clad structure while solution doping is
adopted for
incorporation of the active ion. The steps involved in the process are as
follows:
i. A conventional cladding doped with P205 and F is deposited within a high
silica
3o glass substrate tube to develop matched clad or depressed clad type
structure.
ii. The core layers of predetermined composition containing index-raising
dopant like
Ge02 are deposited at a lower temperature to form unsintered porous soot.
iii. The tube with the deposit is immersed into an aqueous solution of the
dopant
precursor (typical concentration 0.1 M) up to 1 hour. Any soluble form of the
dopant ion is suitable for preparation of the solution although rare earth
halides
have been mostly used.
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iv. Following immersion, the tube is rinsed with acetone and remounted on
lathe.
v. The core layer containing the RE is dehydrated and sintered to produce a
clear
glassy layer. Dehydration is carried out a temperature of 600°C by
using chlorine.
The level of OH- is reduced below lppm using C12 / OZ ratio of 5:2 provided
the
drying time exceeds 30 min.
vi. Collapsing in the usual manner to produce a solid glass rod called
preform.
vii. Fibre drawing is conventional.
Reference may also be made to DiGiovanni D.J., SPIE Vol. 1373 (1990) p-2
"Fabrication
of rare-earth-doped optical fibre' wherein the substrate tube with the porous
core layer is
to soaked in an aqueous or alcoholic solution containing a nitrate or chloride
of the desired
RE ion. The tube is drained, dried and remounted on lathe. The dehydration is
carried out
by flowing dry chlorine through the tube at about 900°C for an hour.
After dehydration, the
layer is sintered and the tube is collapsed to be drawn to fibre.
Another reference may be made to Ainslie B.J., Craig S.P., Davey S.T., and
Wakefield B.,
Material Letters, Vol. 6, (1988) p-139, " The fabrication, assessment and
optical properties
of high- concentration Nd3+ and Er3+ doped silica based fibres" wherein
optical fibres
based on A1203 - PZOS. - Si02 host glass doped with high concentrations of
Nd3+ and Er3+
have been fabricated by solution method and quantified. Following the
deposition of
cladding layers P205 doped silica soot is deposited at lower temperature. The
prepared
2o tubes are soaked in an alcoholic solution of 1M Al(NO3)3 + various
concentrations of ErCl3
and NdCl3 for 1 hour. The tubes are subsequently blown dry and collapsed to
make
preforms in the usual way. A1 is said to be a key component in producing high
RE
concentrations in the core centre without clustering effect. It is further
disclosed that Al
and RE profile lock together in some way which retards the volatility of RE
ion. The dip at
the core centre is observed both for P and GeOa.
Reference may also be made to US Patent No. 5,005,175 (1991) by Desuvire et
al.,
'Erbium doped fibre amplifier" wherein the fibre for the optical amplifer
comprises a
single mode fibre doped with erbium in the core having a distribution profile
of the RE ion
whose radius is less than 1.9 ~,m while the radius of the mode of the pump
signal exceeds 3
~.m. The numerical aperture (NA) of the fibres varies from 0.2 to 0.35 and the
core is
doped with both A1 and Ge oxides to increase the efficiency. As the radius of
the Er doped
core region is equal to or less than the radius of the pump mode of the fibre
it is claimed
that each atom of erbium in the core cross section is exposed to substantially
equal levels
of the high intensity portion of the pump mode. The fibre with such design is
reported to
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a v~ ~ ~ ~ r .,. . - ~e . - '.
have increased gain and lower threshold compared to the conventional Er doped
fibre
amplifiers where the radius of the Er doped core is large compared to the
radius of the
pump mode so that the erbium atoms at the edge of the core do not see a
sufficient flux of
the pump photons to yield a net gain.
According to US Patent No. 5,491,581 (1996) by G.S. Roba, 'Rare earth doped
optical
fibre amplifiers' wherein high germania concentration in the core used to
enhance the NA
of the fibre is reported to result in generation of residual stress at the
core-clad interface
due to difference in viscosity and thermal expansion coefficient. Residual
stress in turn is
believed to produce undesirable increase in background loss of the fibre.
According to US Patent No. 5,778,129 (1998) by Shukunami et.al., 'Doped
optical fibre
having core and clad structure are used for increasing the amplification band
of an optical
amplifier using the optical fibre' wherein the porous core layer is deposited
after
developing the cladding inside a quartz tube by MCVD process and solution
doping
method is employed to impregnate Er as the active ion into the porous core to
be followed
by vitrification and collapsing for making the preform. The solution also
contains
compound of Al, say chlorides, for codoping of the core with Al in order to
expand the
amplification band. The Er and Al doped glass constitutes first region of the
core.
Surrounding this are the second and third regions of the core. The third
region contains Ge
to increase the refractive index. The second region has an impurity
concentration lower
2o than both those of first and third regions and consequently low RI also.
The second region
acts as a barrier to prevent diffusion of the active dopant.
Reference may also be made to US Patent No. 5474588 (1995) by Tanaka, D. et.
al.,
'Solution doping of a silica with erbium, aluminium and phosphorus to form an
optical
fibre' wherein a manufacturing method for Er doped silica is described in
which silica
glass soot is deposited on a seed rod ( VAD apparatus ) to form a porous soot
preform,
dipping the said preform into an ethanol solution containing an erbium
compound, an
aluminium compound and a phosphoric ester, and desiccating said preform to
form Er, Al
and P containing soot preform. The desiccation is carried out for a period
ofe24 -240 hours
at a temperature of 60° - 70°C in an atmosphere of nitrogen gas
or inert gas. This
desiccated soot preform is heated and dehydrated for a period of 2.5 - 3.5
hours at a
temperature of 950° - 1050°C in an atmosphere of helium gas
containing 0.25 to 0.35%
chlorine gas and further heated for a period of 3-5 hours at a temperature of
1400 ° -
1600°C to render it transparent, thereby forming an erbium doped glass
preform. The
segregation of AlCl3 in the preform formation process is suppressed due to the
presence of
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phosphorus and as a result the doping concentration of A1 ions can be set to a
high level
>3 wt%). The dopant concentration and component ratio of Er, A1 and P ions are
claimed
to be extremely accurate and homogeneous in the radial as well as in
longitudinal
directions.
A few of the drawbacks of the above mentioned processes are as follows:
1. Step like RE distribution profile is obtained in the core resulting to poor
overlap
between the pump signal and the RE ions which lowers the pump efficiency.
2., Step like RE distribution requires high numerical aperture (NA) of the
core or
confinement of the RE in the central region (say 50% of the total core area)
for increase
to in pump efficiency which in turn leads to the following disadvantages:
i) Doping of RE 'only in selected portion of the core is extremely difficult
and affects
the repeatability of the process due to the sensitivity of the method to
process
parameters during various stages of processing such as deposition, solution
doping,
drying and sintering.
ii) Increasing the NA of the fibre with simultaneously reducing the core area
requires
high germania concentration in a small core which enhances the possibility of
formation of the dip at the centre due to evaporation during sintering &
collapsing.
iii) For preforms with high NA (>0.20) high germania concentration in the core
lowers
2o the viscosity of the glass and makes the process very sensitive to
temperature
especially during the stages of porous soot layer deposition and sintering.
iv) Increase in temperature sensitivity during porous soot deposition leads to
variation
in composition and soot density along the length of the tube.
v) High germania concentration in the core results to generation of residual
stress at
the core-clad interface due to difference in viscosity and thermal expansion
coefficient. Residual stress produces tmdesirable increase in background loss
of
the fibre.
vi) Residual stress is believed to introduce polarisation mode dispersion
(PMD) which
results in serious capacity impairments including pulse broadening. Since the
3o magnitude of PMD at a given wavelength is not stable passive compensation
becomes impossible.
3. Dehydration and sintering of the RE chloride containing soot layer is
critical because it
alters the composition by vaporisation and also diffusion of the dopant salt
as well as
Ge02 present in the core.
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Objects of the invention
The main obj ect of the present invention is to provide a process for making
Rare Earth
doped optical fibre, which obviates the drawbacks as detailed above.
Another object of the present invention is to provide fibres possessing
controlled
5 distribution of RE, more particularly Erbium in the doped region similar to
the pump
beam intensity distribution in the fibre with maximum concentration at the
centre so that
the overlapping between the two is considerably improved.
Still another object of the present invention is to provide fibres in which
the pump beam
has a radius of distribution equal to or greater than the radius of
distribution of RE ions in
1o the core to increase the chances of all the active ions getting exposed to
the pump light,
consequently increasing the pump conversion efficiency in the fibre.
Yet another object of the present invention is to provide a method of
controlling the
Gaussian RE distribution profile along the radial direction in the core.
Still another object of the present invention is to achieve high optical gain
in the fibres for
NA value close to 0.20 only thus avoiding wide variation in composition
between the core
and cladding glass to eliminate problems like residual stress and PMD.
Yet another object of the present invention is to develop erbium doped fibres
suitable for
amplification of the input signal with NA and mode field diameter not widely
different
from signal transmitting fibre for ease of splice.
2o Still another object of the present invention is to reduce the possibility
of change in
composition of the particulate core layer due to evaporation of the RE salt
during
drying and sintering .
Yet another object of the present invention is to reduce the quantity of
germanium halide
required to achieve the desired NA in the fibre.
One more object of the present invention is to provide a process where the
numerical
aperture of the fibre is varied from 0.10 to 0.30 maintaining RE concentration
in the
core between 50 to 6000 ppm along with variation in RE distribution profile in
the
doped region to produce fibres suitable for application as amplifiers, fibre
lasers and
sensors for different purposes.
3o Summary of the invention
The novelty of the present invention lies in controlling the concentration
profile of RE ion
in the collapsed preform by minimising evaporation of the RE salt and also
preventing
diffusion of the rare earth ion due to subsequent heat treatment. The optimum
soot density
to achieve this objective is estimated to lie between 0.3 to 0.5 after
deposition. The
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inventive step lies in transformation of the RE salts to oxides by gradually
heating the tube
to a higher temperature maintaining an oxidising atmosphere inside, thereby
minimising
the possibility of evaporation of RE during subsequent processing as the oxide
has a very
high melting temperature compared to halide/nitrate salts. This step also
helps to remove
the solvent trapped within the porous layer. The inventive step also includes
increasing the
temperature of the RE containing porous layer gradually in steps of 50 to
200°C up to the
sintering temperature and above for sintering and further fixing of the RE
ions in their
desired sites. The steps will depend on the host glass composition and Er/Al
concentration
of the core layer. The incorporation efficiency of the RE from the solution to
the core
l0 layer thus increases appreciably making the process more efficient and
economic. The RE
distribution along the transverse direction in the core will depend on the
density of the
porous soot layer, dipping period and the processing conditions during
oxidation, sintering
and collapsing.
The sintering of the porous core layer in Ge02 rich atmosphere along with the
addition of
oxygen and helium is another inventive step of the process which reduces the
quantity of
GeCl4 required to achieve the desired NA and adds to the economy of the
process. At
temperatures between 200° to 1400° C during the sintering step
pure GeCl4 is supplied with
the input oxygen, the quantity of which depends on the NA desired in the
fibre. The
sintering is continued by gradually raising the temperature till a clear
glassy layer is
formed.
Detailed Description of the Invention
Accordingly the present invention provides an improved process for making rare
earth
doped optical fibre which comprises (a) providing deposition of P205 and F
doped
synthetic cladding within a silica glass substrate tube to obtain matched or
depressed clad
type structure, (b) forming a core by depositing unsintered particulate layer
at a tube
surface temperature in the range of 1200-1400°C, (c) maintaining Pa05
and Ge02
concentrations from 0.5 to 5.0 mol% and 3.0 to 25.0 mol% in the said
particulate layer
respectively to obtain a tube containing F-doped cladding and porous soot
layer, (d)
immersing the tube containing the porous soot layer into a solution containing
RE salt in
3o the concentration range of 0.002M to 0.25 M with or without aluminium salt
of the
concentration range 0.05 M to 1.25 M for a period of 1 to 2 hours, (e)
draining the solution
out at a rate in the range of 10-50 cc/min, (f) drying the porous layer by
flowing dry
nitrogen or any other inert gas through the tube, (g) heating the tube
gradually in the
presence of oxygen at a temperature in the range of 600-1100°C, (h)
dehydrating the core
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layer of the tube at a temperature in the range of 800-1200°C and in
presence of excess
C12, (i) sintering the core layer in the presence of a mixture of oxygen and
helium in the
temperature range of 1400 to 1900°C, (j) collapsing the tube at a
temperature in the range
of 2000-2300°C to obtain a preform, (k) overcladding the preform with
silica tube, (1)
drawing fibres from the preform.
The present invention further provides an process for making erbium doped
optical fibre
which comprises (a) providing deposition of P205 and F doped synthetic
cladding within a
silica glass substrate tube to obtain matched or depressed clad type
structure, (b) forming a
core by depositing unsintered particulate layer at a tube surface temperature
in the range of
l0 1200-1350°C, (c) maintaining P205 and Ge02 concentrations from 0.5
to 3.5 mol% and 3.0
to 20.0 mol% in the said particulate layer respectively to obtain a tube
containing F-doped
cladding and porous soot layer, (d) immersing the tube containing the porous
soot layer
into a solution containing Er salt in the concentration range of 0.004M to
0.20 M with or
without aluminium salt at the concentration range of 0.05 M to 1.0 M for a
period of 1 to
2 hours, (e) draining the solution out at a rate in the range of 10-30 cc/min,
(f) drying the
porous layer by flowing dry nitrogen through or any other inert gas the tube,
(g) heating
the tube gradually in the presence of oxygen in the temperature range of 700-
1000°C , (h)
dehydrating the core layer of the tube at a temperature in the range of 800-
1200 °C and in
presence of excess Cla, (i) sintering the core layer in the presence of a
mixture of oxygen
and helium in the temperature range of 1400 to 1800°C, (j) collapsing
the tube at a
temperature in the range of 2000-2300°C to obtain a preform, (k)
overcladding the preform
with silica tube, and (1) drawing fibres from the preform.
The present invention also provides an process for making rare earth doped
optical fibre
wherein the RE distribution along the transverse direction in the core is
varied by
controlling the density of the porous soot layer, dipping period and the
processing
conditions during oxidation, sintering and collapsing depending on the host
glass
composition and RE/Al concentration of the core layer. The numerical aperture
of the fibre
is varied from 0.10 to 0.30 maintaining RE concentration in the core between
50 to 6000
ppm along with variation in RE distribution profile along the radial direction
in the doped
region to produce fibres suitable for application as amplifiers, fibre lasers
and sensors for
different purposes.
In an embodiment of the present invention, theoretically estimated relative
density of the
porous soot ranges between 0.30 to 0.50 to avoid core-clad interface defect.
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In another embodiment of the present invention GeCl4 supplied during soot
deposition is
to 30% less than that required for achieving the desired NA numerical
aperture.
In another embodiment of the invention, the pump beam has a radius of
distribution equal
to or greater than the radius of distribution of Er ions in the core, which
enhances the
5 chance of all the active ions getting exposed to the pump light.
In another embodiment of the invention, relatively high gain is achieved in
the fibres for
NA (Numerical aperture) value close to 0.20.
In yet another embodiment of the present invention RE salt used is selected
from chloride,
nitrate or any other salt soluble in solvent used in the process.
to In still another embodiment of the present invention aluminium salt used is
selected from
chloride, nitrate or any other salt soluble in solvent used in the process.
In yet another embodiment of the present invention solution for aluminium and
RE salts is
prepared using solvent selected from alcohol and water.
In still another embodiment of the present invention the temperature of the
core layer is
increased in steps of 50 to 200°C during oxidation and sintering
depending on the
composition and Al/RE concentration of the core layer.
In yet another embodiment of the present invention the mixture of 02 and He is
in the
range of 3:1 to 9:1.
In still another embodiment of the present invention source of chlorine is
selected from
2o CC14 where Helium is used as carrier gas.
In yet another embodiment of the present invention the proportion of Clz: 02
varies from
1.5: 1 to 3.5: 1 while the dehydration period lies between 1 to 2 hours.
In yet another embodiment of the present invention the porous core is sintered
in presence
of germania by supplying GeCl4 with the input oxygen at a temperature of
1200°C to
1400°C during sintering to facilitate germania incorporation and obtain
appropriate
numerical aperture.
In yet another embodiment, the process provides variation in the numerical
aperture of the
fibre from 0.10 to 0.30 maintaining RE concentration in the core between 50 to
6000 ppm
along with variation in RE distribution profile along the transverse direction
in the doped
3o region to produce fibres suitable for application in any devices.
In yet another embodiment, the devices are amplifiers, fibre lasers and
sensors for different
purposes where optical fibre is used. .
Another embodiment of the invention is a method of controlling the Gaussian RE
distribution profile along the radial direction in a core used in the process
of making rare
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earth doped optical fibre wherein, said process comprising the steps of
a) Depositing P205 and F within a high silica glass substrate tube to make
matched
clad or depressed clad type structure.
b) Depositing predefined composition of unsintered particulate Iayer at a
temperature of 1200 to 1400°C for the forming a core, wherein P205 and
Ge02
levels in the core vary from 0.5 to 5.0 mol% and 3.0 to 25.0 mol%
respectively,
and GeCl4 concentration in the gas phase is kept 10 to 30% lower than that
required for achieving the desired NA of 0.20.
c) The deposition temperature is dependent on the composition and desired
porosity
l0 of the soot. A theoretically estimated porosity of 0.3 to 0.5 is found
suitable to
avoid core-clad interface defect and clustering after dipping and to control
the RE
distribution in the core with maximum concentration at the centre.
d) Immersing the tube containing the porous soot layer into an
alcoholic/aqueous
solution of RECl3 / RE(NO3)3 Of strength varying between 0.002 M and 0.25 M
with or without the addition of AlCl3 / Al(N03)3 in the concentration range
0.05
M to 1.25 M for a period of one to two hours.
e) Draining out the solution slowly at a rate of 10 to 50 cc/min. to avoid
imperfection in the porous soot material, particularly at the lower end of the
tube.
f) Passing dry nitrogen through the tube for sufficient drying of the porous
layer and
2o the tube is remounted on lathe.
g) Repeatedly heating the RE/Al containing particulate layer in the range from
600
to 1100°C (tube surface temperature), in presence of 02+He wherein the
temperature is increased in steps of 50 to 200°C thus oxidising the
RE/Al chloride
or nitrate present in the layer to corresponding oxides, wherein the ratio of
02 &
He is varied between 3:1 to 9:1.
h) The particulate core layer containing RE is dehydrated at a temperature
between
800° to 1200°C in presence of excess chlorine. CC14 is used as
the source material
for C12 and supplied by using Helium as a carrier gas which being a lighter
gas
diffuses through the small pores and assists in the drying process. The
proportion
of C12: 02 varies from 1.5: 1 to 3.5: 1 while the dehydration period lies
between 1
to 2 hours.
i) The porous core layer is then sintered in presence of 02 and He by heating
the
tube to a temperature as high as 1900°C. The temperature is gradually
increased in
steps of 50 to 200°C depending on the composition and RE/Al
concentration of
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j) the core layer from the drying temperature between 800 to 1200°C
mentioned
above.
k) At temperatures between 1200° to 1400°C during sintering pure
GeCl4 is supplied
with the input oxygen to carry out the sintering of the porous layer in
germania
5 rich atmosphere which facilitates germania incorporation. The flow rate of
GeCl4
and the no. of pass depend on the NA desired in the fibre. The supply of GeCl4
is
then stopped and the sintering is continued by gradually raising the
temperature
till a clear glassy layer is formed.
1) The collapsing is carned out at a high temperature (>2000°C) in 3 to
4 passes of
l0 the burner to produce a solid glass rod called preform.
m) The preform is overcladded with silica tubes of suitable dimensions to
achieve the
appropriate core - clad dimensions in the ultimate preform/fibre.
n) Fibres are drawn from the preform.
Brief description of the accompanying drawings
Figure 1 ~ 2 represents Er fluorescence distribution across the fibre core
The invention is further explained with the help of following examples, which
should not
be construed to limit the scope of the invention:
EXAMPLE.1
~ Deposition of F-doped cladding within a silica tube by MCVD process at a
temperature
of1855°C.
~ Unsintered core deposition at a temperature of 1290°C. The carrier
gas flows through
the reagent liquids were adjusted to obtain a composition of SiOa= 90.2 mol%,
PZOs=
1.3 mol% and GeOa= 8.5 mol% in the deposited soot layer.
~ Dipping the tube with the deposited layer in a solution containing 0.025 (M)
ErCl3 and
0.1 S (M) Al(N03)3 9H20 for 1 hour and draining out the solution slowly.
~ Drying by maintaining nitrogen gas flow through the tube for 10 min.
~ Oxidation at temperatures of 725°C, 825°Cand 950°C with
2 passes of the burner at
each temperature maintaining a constant He/OZ ratio of 1:5.
~ Dehydration was carried out at a temperature of 1010°C with a C12: OZ
ratio of 2.5: 1
for a period of 1 hour 15 mins.
~ The temperature was increased in 4 steps up to 1400°C. GeCI4 was
added from this
stage with input oxygen with 3 passes between 1200° 1400°C. The
tube was further
heated to increase the temperature stepwise to 1650°C for complete
sintering of the Er
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& A1 containing porous soot layer. During sintering Oa and He flow was in the
ratio of
4.5:1.
~ The collapsing was done in 3 steps in the usual manner.
~ Overcladding was done to reduce the core:clad ratio to 3.6:125. The NA
measured in
the fibre was 0.204 ~ 0.01.
~ The Er3+ ion concentration in the fibre was 950 ppm with maximum
concentration at
the core centre and distribution as shown in fig.l accompanying this
specification. The
Er distribution in the core was measured from the fibre section by
fluorescence
spectroscopy by Photonics Resource Facility, 60 St. George Street, Suite No.
331,
to Toronto, Ontario, Canada MSS 1A7.
~ The fibre recorded a gain of 35.4 dB. The gain was measured at C-DOT, 39
Main Pusa
Road, New Delhi - 110 005 using their measurement set-up.
EXAMPLE 2
~ Deposition of F-doped cladding inside a silica glass tube by MCVD process at
a
temperature of 1840°C.
~ Unsintered core deposition at a temperature of 1310°C. The carrier
gas flows through
the reagent liquids were adjusted to obtain a composition of SiO2= 9I.6 mol%,
P205=
1.1 mol% and Ge02= 7.3 mol% in the deposited soot layer.
~ Dipping the tube with the deposited layer in a solution containing 0.015 (M)
ErCl3,
6H20 and 0.15 (M) Al(N03)3 9Ha0 for 1.5 hours and draining out the solution
slowly.
~ Drying by maintaining nitrogen gas flow through the tube for 10 min.
~ Oxidation at temperatures of 750°, 800° and 900°C with
2 passes of the burner at each
temperature maintaining a constant He/02 ratio of 1:5.
~ Dehydration was carried out at a temperature of 915°C with a C12: 02
ratio of 2.3: 1 for
a period of one hour.
~ The temperature was increased in 3 steps up to1200°C. GeCl4 was added
from this
stage with input oxygen with one pass each at 1200°, 1300° and
1400°C. The tube was
further heated to increase the temperature stepwise to I6I0°C for
complete sintering of
the Er & AI containing porous soot layer. During sintering 02 and He flow was
in the
ratio of 5:1.
~ The collapsing was done in 3 steps in the usual manner.
~ Overcladding was done to reduce the core:clad ratio to 3.6:125.
~ The NA measured in the fibre was 0.201 ~ 0 .O1.
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~ The Er3+ ion concentration in the fibre was 460 ppm with peak at the core
centre and
similar distribution as shown in accompanying drawings as figure 1.
~ The fibre recorded a gain up to 37 dB as measured from C-DOT, 39 Main Pusa
Road,
New Delhi -110 005 using their measurement set-up.
EXAMPLE 3
~ Deposition of F-doped cladding within a silica tube by MCVD process at a
temperature
of 1870°C.
~ Unsintered core deposition at a temperature of 1250°C. The carrier
gas flows through
l0 the reagent liquids were adjusted to obtain a composition of SiOa= 89.1
mol%, P205=
2.3 mol% and Ge02= 8.6 mol% in the deposited soot layer.
~ Dipping the tube with the deposited layer in a aqueous solution containing
0.07 (M)
ErCl3 and 0.25 (M) Al(N03)3 9H20 for 1 hour and draining out the solution
slowly.
~ Drying by maintaining nitrogen gas flow through the tube for 10 min.
~ Oxidation at temperatures of 730°, 820° and 925°C with
2 passes of the burner at each
temperature maintaining at constant He/OZ ratio of 1:6.
~ Dehydration was carried out at a temperature of 925°C with a C12: 02
ratio is 2.3 : 1 for
a period of 1.5 hour.
~ The temperature was increased in 4 steps up to1400°C. GeCl4 was added
with the input
oxygen with 2 passes at 1200°C and one pass each at 1300°C and
1400°C. The tube
was further heated to increase the temperature stepwise to 1725°C for
complete
sintering of the Er & A1 containing porous soot layer. During sintering 02 and
He flow
was in the ratio of 4:1.
~ The collapsing was done in 3 steps in the usual manner.
~ Overcladding was done to reduce the core:clad ratio to 6.5:125. The NA
measured in
the fibre was 0.22~.O1 .
~ The Er3+ ion concentration in the fibre was 3020 ppm with peak concentration
at the
core centre and Er distribution in the core as shown in accompanying drawing
as figure
-2 measured from the fibre section by fluorescence spectroscopy by Photonics
3o Resource Facility, 60 St. George Street, Suite No. 331, Toronto, Ontario,
Canada MSS-
1 A7.
The main advantages of the present invention are:
1. The developed fibres have a RE distribution in the doped region similar to
the
Gaussian pump beam intensity distribution in the fibre so that the overlapping
between
CA 02436579 2003-08-O1
WO 02/060830 PCT/INO1/00014
13
the two is considerably improved consequently increasing the pump conversion
efficiency in the fibre.
2. The pump beam has a radius of distribution equal to or greater than the
radius of
distribution of RE ions in the core, which enhances the chance of all the
active ions
getting exposed to the pump light.
3. The RE distribution along the transverse direction in the core is varied by
controlling
the density of the porous soot layer, dipping period and the processing
conditions
during oxidation, sintering and collapsing depending on the host glass
composition and
RE/Al concentration of the core layer.
l0 4. The compositions of the core and cladding glass are varied to achieve NA
close to 0.20
for Er3+ ion concentration in the range of 100 to 1500 ppm in order to provide
erbium
doped fibre suitable for pumping for amplification of the input signal with
gain in the
range 10 to 37 dB for optical amplifier application.
5. Wide variation in composition between the core and cladding glass is
avoided due to
relatively low NA in the RE doped fibres mentioned under 4 above eliminating
problems like residual stress and PMD which may substantially degrade the
performance of the fibres.
6. The developed fibres mentioned under 4 and 5 above have NA and mode field
diameter
not widely different from signal transmitting fibre for ease of splice. This
miumises
the optical loss of the signal travelling through the fibres.
7. Sintering in germania rich atmosphere facilitates incorporation of germania
in the core
and reduces the quantity of germanium halide necessary during deposition to
achieve
the desired NA making the process efficient and economic.
8. The oxidation step before drying and sintering of the particulate layer
reduces the
possibility of change in composition due to evaporation of RE salts during
subsequent
processing.
9. The stepwise increase in temperature during oxidation and sintering stages
prevents
diffusion of RE and the codopants minimising the probability of a change in
composition.
10. The incorporation efficiency of RE in the doped region is increased due to
the reason
stated in 8 and 9 above, which adds to the economy of the process.
11. The improvement in process efficiency due to the reasons mentioned in 8 -
10 above
enhances the yield and repeatability of the process.
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14
12. The concentration of RE in the core is varied between 50 to 6000 ppm along
with
variation in RE distribution profile in the doped region and NA between 0.10
to 0.30 to
produce fibres suitable for application as amplifiers, microlasers and sensors
for
different purposes.
